I have noticed that SD40-2 units do upward transition at about 23 mph. Backwards transition happens at about 18 mph. One strange feature is that even if the speed remains above 23 mph, placing the throttle in #1 notch, or idle produces backwards transition. It will not do forward transition, until the throttle goes back above #5 throttle. Why is this required?
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SD40-2 Transition
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On the SD40-2, the Transition Settings is controlled by "E/I" which means Main Generator Voltage/Main Generator Current. The TR11/12 Module receives a scaled down MGV and MGA signal that makes transition happen. The older GE's used a speed input from the Axle Alternator and so does the new SD40-3 units CSX has to control Transition but EMD's use E/I. It just so happens to happen in the area of 23 mph for Forward Transition and like you said backwards Transition is in the area of 18 mph. You also have another feature with the SD40-2 called Transition Delay (TDR) what this does is this, lets say you make Transition and the load is so heavy in Parallel that the unit slows down and makes Backwards Transition and the unit goes back into Series, by this happening the MGV/MGA Limits will change and the new setting will allow you to go in the area of 28 mph before the unit makes Transition again. For this TDR circuit to work, the timing has to be set properly on the TDR relay.
Good technical details. Still, why does reducing throttle to #1 produce backward transition, and why does it have to be advanced to #5 to do upward? This happens at speeds well above the 18-25 mph range. I have wondered about this because my line is a roller coaster. If I tried notching down, to use the grade, there would be not much power, until 5 was reached, and transition happened.
Engineer Spikes, I don't know what else to tell you, it is all controlled by E/I which stands for MGV/MGA on an EMD with a TR11 or TR12 Module. when we calibrate the SD40-2 Locomotives for CSX in Huntington, we have our Locomotives hooked to a Load Cell in Short Circuit, we use what is called an MG Set to simulate the MGV and you have graphs at different settings of MGV & MGA so we can verify that transition is picking up and dropping out at the correct set points. I have been doing it for 38+ years, don't know what else to say. I do enjoy reading your posts on Railroad.net, I have learned a lot off of you.
Thanks for your technical answers. The weird backwards transition in #1 has always bugged me. If i have to drop the throttle that low, while at high speed, there is no real power, until #5 is reached, and upward transition happens. I just have to adjust my running to compensate, just like notching out early, on a over glorified washing machine. This is to compensate for slow loading.
That was an appreciated compliment, WVU.
That was an appreciated compliment, WVU.